An Observational and Modeling Study of Mesoscale Airmasses with High Theta-E

Typically, the cool side of the outflow boundary is stable to vertical motions due to the high density and low buoyancy of the air. However, under certain instances, the air on the cool side of the boundary can undergo a transition where it obtains a higher equivalent potential temperature and thus, higher surface-based convective available potential energy than the airmass on the warm side of the boundary. This phenomenon will hereafter be referred to as a mesoscale airmass with high theta-e, or MAHTE. Although they are on the meso-γ scale, MAHTE can have significant impacts on storms which interact with them. Examples of MAHTE were documented during the VORTEX-95 and the BAMEX field campaigns. In both cases, storms which interacted with the MAHTE rapidly strengthened, producing more pronounced low-level mesocyclones, tornadoes, and record-breaking hail. Due to the locally enhanced conditional instability, understanding the processes responsible for MAHTE formation and evolution are important for forecasters to properly assess the probability of severe weather potential.

The authors have undertaken an observational and mesoscale modeling study to characterize and examine the processes responsible for the formation and evolution of MAHTE. Surface observations of a MAHTE in northwestern Kansas on 20 June 2016 were collected across the MAHTE with an Integrated Mesonet and Tracker (IMeT). Surface observations showed the highest values of equivalent potential temperature in all transects were within 1 – 8 km on the cool side of the boundary. Theta-e values in the MAHTE were 10 – 15 K higher than what was observed in the warm sector. This case was modeled using WRF-ARW to examine the processes involved in MAHTE formation that could not be inferred through observations alone. Additionally, the two aforementioned MAHTE cases were modeled in WRF-ARW for a complete analysis of MAHTE. Model analysis indicated that differential vertical mixing across the boundary was important for MAHTE formation. Greater and deeper vertical mixing in the warm sector reduced moisture, while vertical motions were suppressed on the cool side of the boundary. Model analysis also suggested that surface fluxes did not play a major role in MAHTE formation.